U.S. patent number 10,284,050 [Application Number 15/737,975] was granted by the patent office on 2019-05-07 for apparatus for avoiding harmful bearing currents.
This patent grant is currently assigned to ebm-papst-Mulfingen GmbH & Co. KG. The grantee listed for this patent is ebm-papst Mulfingen GmbH & Co. KG. Invention is credited to Roland Oberst, Malte Pils, Sebastian Schroth, Marco Weckert.
United States Patent |
10,284,050 |
Schroth , et al. |
May 7, 2019 |
Apparatus for avoiding harmful bearing currents
Abstract
The present invention relates to an apparatus for reducing
and/or avoiding harmful bearing currents in an electrical machine
(M) such as preferably a three-phase EC motor (M), with a rotor (2)
and a stator (3) which is constructed in an insulated manner,
wherein at least one outer bearing ring (4a) and one inner bearing
ring (4b) are provided between rotor (2) and stator (3), comprising
connecting electronics (10) for connecting the motor (M), wherein
the stator (3) is connected by means of an electrical connection
(20) to a high-frequency electronics potential (11) which differs
from the earth potential (U.sub.E) and is stable with respect to
the latter at a potential tap (20a) of the connecting electronics
(10).
Inventors: |
Schroth; Sebastian (Kupferzell,
DE), Oberst; Roland (Gruensfeld, DE), Pils;
Malte (Ilshofen-Oberaspach, DE), Weckert; Marco
(Doerzbach-Hohebach, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
ebm-papst Mulfingen GmbH & Co. KG |
Mulfingen |
N/A |
DE |
|
|
Assignee: |
ebm-papst-Mulfingen GmbH & Co.
KG (Mulfingen, DE)
|
Family
ID: |
55434009 |
Appl.
No.: |
15/737,975 |
Filed: |
May 4, 2016 |
PCT
Filed: |
May 04, 2016 |
PCT No.: |
PCT/EP2016/059990 |
371(c)(1),(2),(4) Date: |
December 19, 2017 |
PCT
Pub. No.: |
WO2017/016692 |
PCT
Pub. Date: |
February 02, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180301953 A1 |
Oct 18, 2018 |
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Foreign Application Priority Data
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Jul 24, 2015 [DE] |
|
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10 2015 112 146 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02P
27/06 (20130101); H02K 11/40 (20160101); H02K
5/1732 (20130101); H02K 11/20 (20160101) |
Current International
Class: |
H02K
11/20 (20160101); H02K 5/173 (20060101); H02P
27/06 (20060101); H02K 11/40 (20160101) |
Field of
Search: |
;310/40R,71,72,68D,68R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10 2004 016738 |
|
Nov 2005 |
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DE |
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10 2006 007437 |
|
Aug 2007 |
|
DE |
|
1 445 850 |
|
Aug 2004 |
|
EP |
|
Other References
International Search Report dated Aug. 10, 2016 which issued in PCT
Patent Application No. PCT/EP2016/059990. cited by applicant .
Sebastian Schroth et al., "Impact of Stator Grounding in Low Power
Single-Phase EC-Motors", 2014 IEEE Applied Power Electronics
Conference and Exposition--APEC 2014, IEEE, Mar. 16, 2014 (Mar. 16,
2014), pp. 783-790. cited by applicant.
|
Primary Examiner: Andrews; Michael
Attorney, Agent or Firm: Dickinson Wright PLLC
Claims
The invention claimed is:
1. An apparatus (1) for reducing and/or avoiding harmful bearing
currents in an electrical machine (M) comprising a three-phase EC
motor (M), with a rotor (2) and a stator (3) which is constructed
in an insulated manner, wherein at least one outer bearing ring
(4a) and one inner bearing ring (4b) are provided between rotor (2)
and stator (3), comprising connecting electronics (10) for
connecting the motor (M), wherein the stator (3) is connected by
means of an electrical connection (20) to a high-frequency
electronics potential (11) which differs from the earth potential
(U.sub.E) and is stable with respect to the latter at a potential
tap (20a) of the connecting electronics (10); characterized in that
the connecting electronics (10) comprises a rectifier (14), an
intermediate circuit (15) and an inverter (16), and the potential
tap (20a) in the connecting electronics (10) is provided so that
the electronics potential (11) connected to the stator (3)
corresponds to a potential of the intermediate circuit (15), to the
positive intermediate circuit potential (DC+), to a ground
potential of the intermediate circuit (15), to a potential at the
center tap (15') of the intermediate circuit (15) or to the
negative intermediate circuit potential (DC-).
2. The apparatus (1) according to claim 1, characterized in that
the electrical connection (20) between the stator (3) and the
electronics potential (11) is formed by an intermediate capacitance
(21) or via an intermediate impedance.
3. The apparatus (1) according to claim 1, characterized in that
the electrical connection (20) between the stator (3) and the
electronics potential (U.sub.E) is formed as an electrical short
circuit connection.
4. The apparatus (1) according to claim 1, characterized in that
the potential tap (20a) for the stator (3) is a tap immediately
before or immediately after the rectifier (14).
5. The apparatus (1) according to claim 1, characterized in that
the connecting electronics (10) moreover comprises an EMC filter
(17) on the input side, and the potential tap (20a) for the stator
(3) is a tap within the EMC filter (17).
6. The apparatus (1) according to claim 1, characterized in that
the connecting electronics (10) is connected to a low-voltage power
supply or switching power supply, and the potential tap (20a) for
the stator (3) is provided as a tap at the outputs of the
low-voltage power source or of the switching power supply.
7. The apparatus (1) according to claim 1, characterized in that
the rectifier (14) is an active or passive rectifier.
8. The apparatus (1) according to any one of claim 1, characterized
in that a network which reduces impedance in the frequency range of
the bearing voltage is connected in parallel to at least one of
diodes (14a) of the rectifier (14).
9. An electric motor (M), preferably an EC motor, designed with an
apparatus (1) according to claim 1.
Description
The present invention relates to an apparatus for reducing and in
particular for avoiding harmful bearing currents in an electrical
machine such as an EC motor as well as to an electrical machine
equipped with such an apparatus.
Variable speed motors are supplied today primarily by voltage
source inverters. However, the supply through the voltage source
inverter leads to bearing currents in the bearings of the
motor.
Such a current flow through the bearing can lead to damage
including total failure in electrical machines with rolling and
sliding bearings. In grid operation, so-called ripple voltages can
occur which are induced in a conductor loop which consists of the
shaft, the two bearings, the end plates as well as the housing.
The background and physical causes of ripple voltages are described
extensively in the literature. Expressed simply, due to asymmetries
of the magnetic circuit, magnetic flows that do not add up to
"zero" occur within the electrical machine, causing said annular
magnetic flow.
For the generation of the required current form, an inverter
circuit (inverter) is used, which by PWM on average applies a
voltage to the terminals of the EC motor such that the desired
current form is obtained. The inverter here works typically with a
switching frequency outside of the audible range (>16 kHz).
If the supply of the electrical machine is generated from a pulse
width modulated a second type of electrical bearing stress occurs,
namely a capacitively coupled-in bearing voltage. Due to the
switched pulse pattern of the inverter, a common mode voltage
relative to the earth potential is obtained at its output, which
undergoes large jumps with the switching frequency of the inverter.
This common mode voltage is also transferred via capacitive
coupling networks to the bearing. The physical cause of this type
of bearing voltage consists of the voltage changes at the inverter
output, which are generated by the rapid switching processes and by
the control method of the pulse width modulated inverter which is
in direct correlation with the common mode voltage.
As a result, a voltage is generated between the inner bearing ring
and outer bearing ring and the ball of the bearing running on an
insulating oil film, wherein the oil film between inner bearing
ring and outer bearing ring can be considered to be a capacitor
from the electrical standpoint. If, due to insufficient insulation
strength of the oil film or excessive bearing voltages, a breakdown
of the insulation then occurs, the oil film capacitance is
discharged, and a compensation of the charge bearer between inner
bearing ring and outer bearing ring (electric discharge machining)
occurs.
Depending on the design of the motor, such bearing currents can
lead to a premature failure of the bearing, which leads to a ripple
formation on the running surfaces of the bearing and to
decomposition of the bearing grease.
To remedy this, in the past, current-insulated bearings have been
used, for example, bearings with a ceramic insulation on the outer
ring or a hybrid layer with ceramic roller bodies. However, since
these bearings are very expensive, such a solution is not ideally
suited for mass production.
From the prior art, other remedies are known. Thus, the teaching of
published documents EP 1 445 850 A1 or DE 10 2004 016 738 B3 is to
use a device for protecting a bearing of an electrical machine,
which provides a compensation circuit or a compensation device for
generating a compensation current for the compensation of a
parasitic current through the bearing.
An alternative solution is achieved by a targeted short circuit
between the bearing rings. The teaching of US 20080088187A1, for
example, is to provide a conductive connection between rotor and
stator and to implement a short circuit between rotor and stator
via a spring. Alternatives to this are also known, in which a
bypass element (for example, bypass capacitor) between the bearings
is used.
The disadvantages here include that the stator and rotor are
connected in a conductive manner to one another. Thus, the required
insulation between windings and the stator has to be
correspondingly strengthened. Furthermore, wear phenomena occur on
the sliding contact, and an additional bypass element also brings
about additional costs.
Another known measure consists in insulating the shaft and
overmolding the stator. Due to the additional insulation layer
between the shaft and the stator, part of the bearing voltage is
reduced by the insulation. Here, it is disadvantageous that the
bearing voltages occurring can nevertheless be sufficiently high so
that undesired bearing currents occur.
A solution such as impedance matching of the end plate, disclosed,
for example, in US 2011/0234026 A1, is only suitable for designs in
which conductive end plates are provided in the first place.
From the publication by S. Schroth, D. Bortis, J. W. Kolar "Impact
of Stator Grounding in Low Power Single-Phase ECMotors, Proceedings
of the 29th Applied Power Electronics Conference and Exposition
(APEC 2014), Texas, Houston, USA, Mar. 16-20, 2014," a solution is
known, in which the stator has an insulating design, and there is
capacitive connection of the stator to the earth potential. Here,
it is problematic that the connection of the stator to the earth
potential has an effect on the filtering properties of the EMC
filter and leads to a clear worsening of the EMC properties, since,
at the same time, the impedance with respect to the earth is
decreased. This results in a clearly increased filter volume. This
means that, in each case, an adaptation to the motor has to occur
individually, so that such a solution can also not be used
universally.
Therefore, the underlying aim of the invention is to provide a
solution by means of which the undesired bearing currents can be
effectively reduced or prevented entirely, in which the
above-mentioned disadvantages do not occur, and which are
universally usable.
This aim is achieved by means of an apparatus for preventing
harmful bearing currents in an electrical machine having the
features of Claim 1 as well as by means of a motor having the
features of Claim 9.
The basic idea of the invention is that the protection of the
bearing against undesired bearing currents occurs by connecting the
stator to a high-frequency potential, preferably the electronics
potential, that is stable with respect to the earth potential
wherein the connection occurs either directly as a short circuit
connection or alternatively via a capacitance or an impedance,
whereby, at the same time, the insulation extent required for
protection against contact is ensured by the introduced
impedance.
The electrical connection of the stator to a stable electronics
potential via any desired impedance can accordingly be implemented
in a resistive, inductive or capacitive manner or by an integration
of entire grid topologies.
In order to enable motor operation with variable speed operation of
a motor, the motors are supplied via inverters with corresponding
high cycle frequency. A typical current rectifier with DC
intermediate circuit (also referred to as voltage source inverter)
usually consists of a rectifier, the intermediate circuit with
constant voltage and an inverter, wherein a three-phase voltage
system is generated from the constant intermediate circuit voltage.
The intermediate circuit voltage is referenced to the earth
potential via the rectifier and/or the EMC filter. Due to the
pulsing, a voltage occurs between the earth potential and the motor
phases, which corresponds to half the intermediate circuit voltage
(.+-.U.sub.ZK/2). This voltage occurs in all three phases as in
phase common mode voltage.
According to the invention, the above-mentioned stable potential of
the electronics to which the stator is electrically connected is a
potential of a preferably filtered voltage source inverter, which
is different from the grounding potential. Thus the connection can
be to the intermediate circuit potential DC+ of the intermediate
circuit or the ground potential of the electronics or DC-, to the
potential before or after the rectifier of the rectifier circuit,
to the potential within the EMC filter, or alternatively also the
output potential of the low-voltage power supply or of the
switching power supply.
According to the invention, an apparatus for reducing and/or
avoiding harmful bearing currents in an electrical machine is
proposed, such as, for example, in a three-phase EC motor, with a
rotor and a stator which is constructed in an insulated manner,
wherein an outer bearing ring and an inner bearing ring are
provided between rotor and stator, comprising moreover connecting
electronics for connecting the motor, wherein the stator is
connected by means of an electrical connection to a high-frequency
electronics potential which differs from the earth potential and is
stable with respect to said earth potential at a potential tap of
the connecting electronics.
In a preferred embodiment of the invention, it is provided that the
electrical connection between the stator and the electronics
potential is formed via an intermediate capacitance or via any
intermediate impedance.
Alternatively, it can be provided that the electrical connection
between the stator and the electronics potential is formed as an
electrical short circuit connection. Thus, during the operation of
the motor, the stator is directly at the electronics potential.
In another advantageous design of the apparatus according to the
invention, it is provided that the connecting electronics comprises
at least one rectifier, an intermediate circuit and an inverter,
and the potential tap for the connection of the stator is arranged
in the connecting electronics, so that the electronics potential
connected at the stator corresponds to one of the potentials
mentioned below: the potential of the intermediate circuit,
preferably the positive intermediate circuit potential (DC+) the
ground potential of the intermediate circuit (15), the potential at
the center tap (15') of the intermediate circuit (15) or the
negative intermediate circuit potential (DC-).
Alternatively, the potential tap for connecting the stator to the
electronics potential can also be a tap directly before or after
the rectifier. In this case, in a development of the invention, it
is provided that a network which reduces the impedance in the
frequency range of the bearing voltage is connected in parallel to
a diode, to several diodes or to all the diodes of the
rectifier.
In another alternative design of the invention, the connecting
electronics moreover comprises on the input side an EMC filter, and
the potential tap for the stator is a tap within the EMC
filter.
Moreover, it can be provided alternatively that, to the extent that
the connecting electronics is connected to a low-voltage power
supply or to a switching power supply, the potential tap for the
stator is provided as a tap at the outputs of the low-voltage power
supply or of the switching power supply.
Another aspect of the present invention relates to an electric
motor, preferably to an EC motor, which is designed with an
apparatus as described before.
Other advantageous developments of the invention are characterized
in the dependent claims or represented in further detail below
together with the description of the preferred design of the
invention in reference to the figures.
FIG. 1 shows a representation of the whole system in the case of a
connection of the stator to the electronics potential,
FIG. 2 shows a modeling of the capacitance network,
FIG. 3 shows an equivalent circuit diagram of the parasitic bearing
capacitances and the main current path of the bearing current,
FIG. 4 shows an equivalent circuit diagram of the parasitic bearing
capacitances in the connection of the stator with the consequently
redirected earth current path,
FIG. 5 shows a diagrammatically simplified equivalent circuit
diagram for determining the bearing voltage according to the design
of FIG. 3,
FIG. 6 shows comparative measurement curves of the common mode
voltage and of the bearing voltage (without and with connection of
the stator),
FIG. 7 shows an alternative embodiment of an apparatus according to
the invention,
FIG. 8 shows another alternative embodiment of an apparatus
according to the invention in a second variant,
FIG. 9 shows a second alternative embodiment of an apparatus
according to the invention in the second variant,
FIG. 10 shows a third alternative embodiment of an apparatus
according to the invention in the second variant,
FIG. 11 shows a fourth alternative embodiment of an apparatus
according to the invention in the second variant,
FIG. 12 shows another alternative embodiment in an apparatus
according to the invention in a third variant,
FIG. 13 shows other alternative embodiments of an apparatus
according to the invention in the third variant.
The invention is described below based on preferred embodiment
examples in reference to FIGS. 1 to 13, wherein the same reference
numerals mark functionally and/or structurally identical
features.
In FIG. 1, a representation of an embodiment example of a whole
system is shown, in which an apparatus 1 for reducing and/or
avoiding harmful bearing currents in the three-phase supplied EC
motor M is represented. The motor M is designed with a rotor 2 and
with a stator 3 which is constructed in an insulated manner, which,
in the case at hand, are represented only diagrammatically.
The apparatus 1 comprises (as can be seen further in FIG. 1)
connecting electronics 10 for connecting the motor M, with a
rectifier 14, an intermediate circuit 15 and an inverter 16 as well
as, on input side, with an EMC filter 17. As can also be seen in
FIG. 1, the stator 3 is connected via a connection 20 to the
potential tap 20a on the inverter 16.
To explain the operating mode of the apparatus according to the
invention, below in FIG. 2, a modeling of the 3-phase inverter 16
as well as of the capacitive network of the parasitic capacitances
is represented.
On the connection side, the intermediate circuit voltage DC+, DC-
of the intermediate circuit 15 (as represented in FIG. 1) is in
applied to the inverter 16. The voltages applied to the motor
terminals are marked with u.sub.u, u.sub.v and u.sub.w, while the
half bridges are represented using the simplified half-bridge
symbol.
The voltage applied by the inverter 16 to the motor terminals is
decomposed into a common mode portion (u.sub.CM) and a differential
mode portion (u.sub.DM). The common mode portion is composed of the
average of the voltage provided by the half-bridges of the
inverter. The differential mode voltage is the difference between
the voltage at the switch and the common mode voltage.
In FIG. 3, an equivalent circuit diagram of the parasitic bearing
capacitances with the main current path of the bearing current
through the bearing of the motor M is shown. The rotor 2 is
rotatably mounted relative to the stator 3 via an outer bearing
ring 4a (stator side) and an inner bearing ring 4b (rotor
side).
In a simplified descriptive model, at least the following
capacitances occur in the system, which are marked further in FIG.
3. Due to the small distance between the winding and the stator
packet, a winding-stator capacitance C.sub.WS occurs in the system.
Furthermore, due to the winding heads of the motor, a winding-rotor
capacitance C.sub.WR is also generated in each case.
Between the rotor and the stator, the rotor-stator capacitance
C.sub.RS is located, which is similar to the capacitance of a
cylinder capacitance. Between the stator 3 and the earth, as well
as between the rotor 2 and the earth, respective parasitic
capacitances occur. The capacitive coupling of the iron yoke to the
housing cover or the electronics results in the parasitic
stator-earth capacitance C.sub.SE,
while the parasitic rotor-earth capacitance C.sub.RE is typically
caused by extensions on the rotor (for example, metal impellers).
The bearing capacitance between the outer bearing ring 4a and the
inner bearing ring 4b is marked with C.sub.bearing.
A change in the neutral point potential of the C.sub.WS/C.sub.WR
star circuit can be generated only by a change in the common mode
voltage provided by the inverter 16.
Due to the symmetric star circuit of the capacitances from the
winding to the rotor 2 and from the winding to the stator 3
(C.sub.WS and C.sub.WR), the differential mode portion of the
voltage, which is only on the windings, cannot cause any changes in
the neutral point potential. Thus, no transmission of differential
mode voltages into the remaining capacitances of the network
occurs.
The formation of the common mode currents via the capacitive
network moreover requires a closed current path. Said current path
is produced by connecting the inverter 16 to the earth potential
U.sub.E of the earth via the Y capacitors, EMC filters or the grid
installation. The common mode currents thus formed flow from the
inverter 16 to the capacitive network to earth and from there via
the filter components and the grid back to the inverter 16
again.
Typically, the largest proportion here is comprised of the
currents, marked i.sub.EDM in FIG. 3, via the stator 3, via the
outer bearing ring or inner bearing ring, 4a and 4b, respectively,
to the rotor 2 and then to earth (represented with a broken-line
arrow line in FIG. 3).
Due to the different values of the capacitances (C.sub.WS clearly
greater than the capacitance C.sub.WR), the common mode currents
therefore flow mainly via the capacitances C.sub.WS, and in the
case of such a circuit topology, therefore via the bearing rings
4a, 4b.
The larger the additional extensions on the rotor 2 are, the lower
the rotor potential with respect to earth is, and the smaller the
bearing voltages are. However, for the harmful bearing voltages,
only voltage changes .DELTA.U with respect to the earth potential
U.sub.E are relevant.
In FIG. 4, the implementation of the invention is shown based on an
equivalent circuit diagram of the parasitic bearing capacitances in
the connection, i.e., in the connection of the stator 3 via an
impedance Z to the potential tap 20a of the inverter 16. Here, in
the operation of the motor M, the positive intermediate circuit
potential DC+ is applied. As a result, it is ensured that the
potential of the stator 3 is kept constant for high frequencies
with respect to the earth potential U.sub.E, and most of the
currents i do not flow back via the bearing, i.e., the outer
bearing ring 4a or the inner bearing ring 4b, but rather flow again
directly to the intermediate circuit potential.
To the extent that the impedance of Z for the high-frequency
bearing currents can be neglected and the connection 20 between the
stator 3 and the potential tap 20a has short circuit
characteristics, the potential of the stator 3 can also change
maximally with the grid frequency and no longer with the frequency
of the inverter 16. The (simplified) equivalent circuit diagram
resulting in this way can be indicated as in FIG. 5.
Due to the different values of the capacitances (C.sub.WS clearly
greater than the capacitance C.sub.WR), the impedance of the entire
section increases accordingly, and the bearing voltage decreases.
The parasitic capacitance C.sub.RE, which is now parallel to the
bearing, reduces the bearing voltage further, whereby, by means of
this device, a significant reduction of the bearing voltage can be
achieved.
Due to the connection of the stator 3 to the positive intermediate
circuit potential DC+, the amounts of the high-frequency currents
that flow to earth is clearly decreased. This has the advantage
that the volume of the EMC filter is not increased in the solution
according to the invention.
In FIG. 6, comparative measurement curves of the common mode
voltage and of the bearing voltage can be found. The upper image
shows the case without connection of the stator 3, and the lower
image shows the design according to the invention with the
connection 20 between stator 3 and the positive intermediate
circuit potential DC+.
In the upper image, the broken-line shows the pulsed common mode
portion U.sub.CM and the solid line shows the bearing voltage
U.sub.b. In particular in the area at 50 .mu.s, voltage breakdowns
of the bearing voltage can be clearly seen.
In the lower image of FIG. 6, it can be seen that in a stator 3
which is constructed in an insulated manner, as a result of its
connection to the electronics potential, the amplitudes of the
bearing voltage u.sub.b applied to the bearing were significantly
decreased and breakdowns of the bearing voltage on the bearing do
not occur.
In FIGS. 7 to 13, alternative embodiments of the invention are
represented according to two other main variants. The first variant
(shown above) related to the connection of the stator 3 to a
potential tap 20a of the intermediate circuit potential. In FIG. 7,
a solution is shown in which the connection of the stator 3 was
implemented not with the positive but with the negative
intermediate circuit potential DC-. Since only common mode
processes are crucial to the bearing voltage, in the case of
connection of the intermediate circuit 14 to DC-, possibly
occurring bearing currents can be closed via the intermediate
circuit capacitor C.sub.DC.
The intermediate circuit capacitor C.sub.DC, on the other hand, has
capacitances that are clearly higher than the parasitic
capacitances of the motor and can be considered to be a short
circuit, for the sake of simplicity. This results in a direct
connection to the electronics ground, and the potential of the
stator 3 can be kept constant for high frequencies with respect to
the earth.
A second variant relates to any other suitable connection which is
at the potential of the intermediate circuit as shown, for example,
a potential tap 20a at the center tap of the intermediate circuit
15, as in FIG. 8.
A third variant relates to a suitable connection which is at a
potential of the connecting electronics 10, which differs from the
intermediate circuit potential, as shown, for example, in the two
images of FIG. 9, in which the potential tap 20a occurs before the
rectifier 14. When using a Y capacitor, for example, a connection
on the AC side of the rectifier 14 can thus also occur. Such a
configuration would correspond to an alternating connection of the
stator 3 to the ground potential and the intermediate circuit
potential, depending on the sign of the grid voltage currently
applied to the rectifier 14. However, it should be taken into
consideration here that there should be no common mode chokes or
filter elements that strongly increase impedance between the
intermediate circuit 15 and the potential tap 20a, in order not to
prevent the technical effect of the present invention. In this
connection, it should also be taken into consideration that the
given conduction time of the rectifier 14 should be over the
longest possible time interval within the grid half wave. This can
be achieved, for example, by an active PFC circuit. In the case of
a passive rectifier 14, on the other hand, it must be ensured that
either the rectifier 14 has a sufficiently large diode capacitance
or, alternatively, corresponding impedances are connected in
parallel to the rectifier, which reduce the rectifier impedance for
the high-frequency bearing voltage. A corresponding design is
represented in FIG. 10.
Another aspect which should be taken into consideration in the
various embodiments is the protection of the motor M against
contact and thus the protection of persons against electrical
shock, since, due to the connection 20 between the stator 3 and the
electronics potential, the insulation extent is reduced compared to
conventional designs, and, in particular, in a first approximation,
it is reduced to the insulation extent between the bearing seat and
the rotor 2.
One possibility for establishing the required contact protection
consists in providing the connection of the stator 3 via impedances
with sufficient protective insulation. For this purpose, Y
capacitors can be used, for example. For the HF common mode voltage
of the inverter 16, the Y capacitance nevertheless works as a short
circuit and thus like a direct connection. This is the case in
particular if the capacitance used has a clearly smaller impedance
in the frequency range of the bearing voltage than the capacitive
network.
In FIG. 13, other embodiments are represented, which show the
connection 20 of the stator 3 via an impedance Z.
The design of the invention is not limited to the above-indicated
preferred embodiment examples. Instead, many variants that use the
represented solution even in designs of fundamentally different
type are conceivable.
* * * * *